The present invention relates to x-ray inspection apparatus and more particularly to x-ray inspection apparatus which provides an indication of the nature of the materials being inspected, i.e. by measuring line of sight atomic number. More specifically, the invention relates to an apparatus and methods for inspecting cargo held in containers such as trucks or standard shipping containers so as to detect and identify target materials such as explosives and contraband drugs.
Various non-destructive techniques for the inspection of closed containers using x-rays are described in the prior art. For example, U.S. Pat. No. 4,366,382 to Kotowski describes a widely used system for inspecting bags, luggage and the like at airports and buildings. A fan-shaped beam of x-rays irradiates the objects being inspected as they are carried through the beam on a conveyor. A linear array of detectors senses the attenuation produced by the objects on the interrogating x-ray beam and this attenuation information is processed electronically to produce a visual display of the objects. This display is typically referred to as a shadowgraph and while it is highly useful in detecting dense materials, e.g. metallic objects, it does not typically provide any information about the atomic composition of the materials being inspected.
The attenuation of an interrogating x-ray beam is due to the interaction of the x-ray photons with the electrons of the material forming the objects or objects in the path of the interrogating beam. At the low energy levels typically used for baggage inspection, e.g. 160 keV, this interaction is mainly due to both the photoelectric effect and to Compton scattering. In the photoelectric effect, the energy of the incident x-ray photon frees a bound electron. In Compton scattering, the x-ray photon collides with an electron resulting in a loss of energy from the photon with the photon travelling along an altered direction. At low energies, e.g. less than 1 MEV, there is substantial Compton scattering in virtually all directions, i.e. forward, side and back. However, at higher energy levels, e.g. in the order of 4 MEV and higher, the scattering becomes heavily forward biased, that is, the majority of photons are scattered at relatively small angles from the axis of the incident beam.
U.S. Pat. No. 5,247,561 to Kotowski describes a cargo scanning system using x-rays. It uses x-rays mainly in KeV range, but possibly as high as 4.0 MeV. Multiple linear arrays of photodetectors are arranged to detect Compton scattered radiation. Each detector "looks at", and is associated with, one volume element in the target.
A system for providing information regarding the nature of the materials being examined is disclosed in the Neale et al. U.S. Pat. No. 5,524,133. This system utilizes x-rays of two distinct and different energies to obtain additional information regarding the nature of the materials which the x-rays are interrogating. At least one energy is sufficiently high to interact through pair production. The system described employs two different sources which are spatially separated and indifferently oriented so as to avoid interference. The '133 patent also discloses various compound detectors that can be used to infer the spectral hardness of the transmitted beam and from this information infer the average atomic number of the material being inspected.